The javascript Math.rand() is unlikely to be good enough for a lottery, as the specification does not require it to be cryptographically secure. For instance, there are known weaknesses in the generator used in some versions of Chrome.
What you would need would be a cryptographically secure pseudo random number generator (such as blum-blum-shub), and a way of seeding it. You need a good way of seeding it because if anyone can figure out what seed you used, they will be able to generate all the lottery numbers. You would probably want to seed the pseudo-random generator with a genuinely random, rather than a pesudo random number. This would require a hardware random number generator. Random.org supposedly provides a source of genuinely random numbers, however if anyone were to eavesdrop on your connection to random.org, they could still discover your random seed. You might prefer to invest in your own hardware, rather than rely on someone external.
Infact, unless you needed an awful lot of random numbers (much more than 10) there would be little point in using any pseudo random number generator at all. You might as well get all your random numbers from a true, hadware, random number generator.
